In general, there are a number of different approaches to electromagnetic (EM) geophysical exploration, including airborne and ground systems. Airborne systems are generally flown from an aircraft or helicopter by being towed therefrom in a "bird", and generally comprise a number of transmitter and receiver coils operating to transmit primary EM fields and to detect changes in the detected secondary EM fields. In other words, airborne systems are generally useful only for determining the presence of a geophysical anomaly, and are not particularly useful for determining certain parameters of the anomaly whereby the presence of specific commercial ores may be determined, the depth and size of the deposit may be estimated, and other more specific data with respect to particular ores being sought for may be determined. Ground systems may be used for detecting the presence of geophysical anomalies in the first instance, and thereafter some analysis of the geophysical anomaly -- an ore body -- may be determined, including the characteristics mentioned above such as identification of mineralization and metal-carrying ore bodies, the depth and size of the ore body, the possible concentration of specific desired minerals, etc.
In general, prior ground systems which have been used have always required the tuning or rotation, or both, of receiver coils so as to determine certain geophysical data. This has come from the fact that geophysical anomalies which are generally desirable mineralization and ore bodies will produce a secondary alternating electromagnetic field in the presence of a primary alternating electromagnetic field -- and, in general, the primary EM field is transmitted at a frequency in the audio range, e.g. 50 to 20,000 Hz. Where a secondary EM field is produced by a desirable or economic geophysical anomaly, the resultant field is elliptically polarized, and comes as a result of eddy currents which are induced in the geophysical anomaly. Any such resultant field thereby includes signals at the frequency of the transmitted primary EM field which are not in-phase with the transmitted field, and in general such signals are measured with respect to their quantities which can be determined to be "in-phase" and "quadrature" with respect to the transmitted primary field. The resultant EM field, however, is generally tilted with respect to the plane of the transmitted primary field at the location where the receiver is placed, so that it has been necessary in the past, where the tilt angle of the resultant field is desired to be known, to physically rotate or electrically synthesize rotation of the perpendicularly crossed coils in order to find a null point on one of the coils which then gives an indication of tilt angle of the ellipse of polarization of the resultant field. Still further, it has very often been required to use yet a further reference coil in the receiver; and in any event, it has been required that there be some sort of physical or electrical link between the receiver and transmitter, except in those systems which operate either from fixed VLF transmitters or in the presence of magnetic time transients or fluctuations of the earth's magnetic field.
For example, G. H. McLaughlin et al, in U.S. Pat. No. 3,126,510 dated Mar. 24, 1964 teach a method and means of geophysical prospecting where a pair of coils are placed in the magnetic field which normally exists. The fact of the polarization of the normal earth's magnetic field at an angle other than generally horizontal is detected by physically rotating the coils in the field. The coils which are used may be orthogonally related one to the other, and a reference coil may be used; but in any event the rotation of the coil is required.
Another approach to EM prospecting is taught in Seigel U.S. Pat. No. 2,903,642, issued Sept. 8th, 1959, which teaches the use of a pair of transmitting coils which operate at two different frequencies, and a pair of orthogonal receiving coils. The presence of a secondary field, which is indicative of the presence of a geophysical anomaly, is detected and the nature of that anomaly may be determined to some extent by comparison of the orthogonal signals which are detected at both frequencies. The Seigel apparatus requires that the receiver coils are fixed in space with respect to one another, and notes that comparison and continuous recording of the differences of the fields detected in the coils at both frequencies determines the presence of a conductor anomaly. However, there is no consideration of determination of ellipse parameters of the field, merely concern for the detection of a secondary field which is indicative of the presence of a geophysical anomaly.
Ronka, in U.S. Pat. No. 3,500,175 dated Mar. 10, 1970, on the other hand, teaches a portable EM apparatus which utilizes fixed VLF transmitters -- which are operated at at least eight radio stations around the world by the United States Navy and similar authorities -- whereby two coils which are orthogonally fixed with respect one to the other are used. The coils are tilted until a null is observed, which provides an indication of tilt angle of the secondary field which may exist as a result of the VLF signals which are present.
Ghosh et al, in U.S. Pat. No. 3,936,728 issued Feb. 3, 1976 teach an apparatus and method of obtaining diagnostic information with respect to a geophysical anomaly, whereby magnetic induction field and electric induction field components are studied at a series of frequencies in the range of 5 to 43,000 Hz., whereby a reference coil is oriented with respect to the secondary field being studied.
All of these prior approaches to geophysical exploration and particularly to geophysical study of detected anomalies by way of determination of the tilt angle of the ellipse of polarization as well as a measure of the ellipticity angle and ellipticity ratio thereof, have a number of disadvantages including particularly the fact that each measurement takes a considerable period of time because of the necessity of determining a null. Especially when a number of frequencies are used, the measurement time becomes quite uneconomic. In locations where there is a weak signal, or the signal-to-noise ratio is poor, even more time has been required. The accuracy of measurement is, in any event, usually poor and depends to quite an extent on the skill and experience of the operator. Thus, widely different readings may be obtained at the same location by different operators, and analysis of such data then becomes difficult.
Still further, the ground EM systems of the prior art, as discussed above, present the problem that the measurement of ellipticity -- tilt angle, ellipticity angle and ellipticity ratio -- of a detected secondary field is successful, in the first instance, upon the proper orientation of the receiver coils. In the event of minimal ellipticity -- i.e., when the elliptical polarization becomes nearly circular -- it is extremely difficult to distinguish or discriminate null and maximum signal positions or orientations of the coils. Still further, when using conventional ground EM systems, unless particular sign conventions are observed only absolute ellipticity can be determined without regard to the direction of polarization or the direction of tilt angle, thereby resulting in false or misleading data reduction analysis.
The present invention overcomes all of the difficulties spoken of above, and differs significantly from any prior approach, in that it provides a pair of fixed coils which are usually perpendicularly related to each other in a receiver which is located at a place remote from a transmitter and completely independent from the transmitter. No rotation of the coils is required, either physically or by electrical synthesis, and the presence of a geophysical anomaly can be determined by examining the signals on both of the coils, as well as providing for the determination of such ellipse parameters as the tilt angle, ellipticity angle and ellipticity ratio of the polarization ellipse of the resultant EM field.
Thus, the method provided by this invention comprises the steps of fixing a transmitter in a first specific orientation and fixing a receiver which has a pair of crossed coils therein in a second specific orientation, so that the specific orientations of the transmitter and receiver may each be set up at each transmission location and at each testing location, respectively, of the transmitter and receiver. In other words, it is determined that, during any particular electromagnetic survey procedure according to this invention, which may extend over a wide area, there is a specific orientation established for the transmitter and a specific orientation established for the receiver, and each time the transmitter and receiver are set up they are positioned in their respective orientations so that the relative orientation between them remains fixed. The distance between the transmitter and receiver may, of course, vary; and often the transmitter remains in a particular location and is merely rotated so as to maintain a fixed specific orientation with respect to the direction in which the receiver is placed so that the relative orientations between the receiver and transmitter remain constant. The receiver is, of course, placed at a location which is within range of the transmitter but is remote from it; and no wire or radio link is set up between them except as may be established by virtue of the transmission from the transmitter of a primary alternating electromagnetic field. Each time the transmitter and receiver are oriented, a primary alternating EM field is transmitted at at least one known frequency from the transmitter -- and in the usual circumstances, where signals are detected at the receiver, primary EM fields at a number of frequencies are transmitted from the transmitter, one after another, merely by selecting the frequency of operation of the transmitter in a manner discussed in greater detail hereafter. During transmission of the alternating EM field, the orientation of the transmitter and of the receiver are maintained constant, and the output from the coils in the receiver is detected. In general, the crossed coils in the receiver are fixed perpendicularly one to the other; but they may be fixed other than orthogonally, in which case additional signal processing circuits would be required. In any event, when there is an output detected from both of the coils of the receiver, the signals from those coils are examined so as to determine the phase difference between the detected signals and the magnitude ratio of the signals at the frequency of the transmitted primary EM field, which frequency is known. When the phase difference between detected signals of the crossed coils and the receiver and the magnitude ratio of those signals are each determined, the quantities which are expressed thereby may then be mathematically analyzed and processed so as to obtain such ellipse parameters of polarization of the resultant electromagnetic field as the tilt angle, ellipticity angle and ellipticity ratio thereof.
Thus, when a primary alternating electromagnetic field is transmitted at a known frequency, and there is a geophysical anomaly which will manifest itself in the presence of such primary electromagnetic field by producing a secondary electromagnetic field at the same frequency, the total or resultant field is one which is elliptically polarized and whose ellipse of polarization is tilted at any location where the presence of a secondary field is being tested for. In other words, where there is no secondary field, there is no elliptical polarization of a resultant field and therefore signals detected on both coils which are as a result only of the primary field in the absence of any secondary field are of equal magnitude and are in phase. However, in the presence of a secondary field, the resultant field will be elliptically polarized to some extent or another, and thus there will be a phase difference detected between signals which are derived from the pair of crossed coils, respectively; and depending upon the amount of ellipticity, there will be a magnitude ratio between the detected signals from the crossed coils which will be other than unity.
All of this comes because coupling to the crossed coils, in free space, is inductive; and therefore any induced voltage on either coil in free space exists solely as the result of the presence of an electromagnetic field. When there is both a primary and a secondary EM field, the coupling is quite complex in nature, and gives rise to the presence of different signals on the crossed coils -- especially when the crossed coils are perpendicularly related one to the other as would be the usual case -- so that a phase difference and/or a magnitude ratio rather than unity exists with respect to the signals from each of the coils, depending on the amount of ellipticity there is of the resultant EM field.
By the same token, where there is no geophysical anomaly -- i.e., no practical conductive material being tested for at the frequency of the EM field then being transmitted -- there is no electrical polarization of a secondary field at the frequency being transmitted, and no apparent resultant field is therefore detected. Thus, if at any frequency there is an apparent resultant electromagnetic field giving rise to complex coupling to the coils, tests are generally made with transmissions at a number of frequencies for purposes of obtaining more highly diagnostic geophysical data. This comes also as a consequence of the fact that coupling of the geophysical anomaly and the resultant secondary field produced thereby varies depending upon the frequency of the transmitted primary field; and therefore, unless it can be readily determined that there is, indeed, no practical geophysical anomaly present, it is usual to transmit signals at at least several frequencies and to test for a secondary field which is manifested by the complex coupling of the resultant EM field to the fixed and fixedly oriented crossed receiver coils.
During transmission of a primary alternating electromagnetic field from a transmitter at one location to a receiver in another location, the specific orientation of the transmitter and receiver -- and thereby the relative orientation between them -- are maintained; and the signals thereby being sampled are examined to determine the phase difference between them and the magnitude ratio of them. The phase difference between the detected signals is defined as a quantity .phi.; and the magnitude ratio of the detected signals is defined as a quantity r.
When the quantities .phi. and r can be determined at any frequency, those quantities can be mathematically analyzed and processed so as to obtain such ellipse parameters of polarization of the resultant EM field at the transmitted frequency as the tilt angle, ellipticity angle and the ellipticity ratio thereof.
The present invention also provides for the arithmetic addition of the signal level of succeeding signals which are being sampled from both of the crossed coils of the receiver, in respective signal level stacking circuits. This is, in essence, a wave stacking procedure, and may be carried out where there is an output signal from one of the coils having a very low amplitude -- which may be the result of a weak signal component of the resultant field being tested -- or so as to improve the signal-to-noise ratio of the detected signals. In the latter case, the arithmetic addition of the signal level of succeeding signals is gated or otherwise controlled so as to be synchronous with the frequency of the transmitted primary field; e.g. so that each successive positive going signal is added to the signal level of the preceding positive going signals, for example. Because noise is generally quite random in nature, during each successive gating operation of the signal level stacking circuits, the noise may be positive or negative going and, over several cycles of the primary field tends to be self-cancelling. In other words, a noise filter is thereby achieved, especially when the transmitted primary EM field is sinusoidal.
Certain rules can be established for determination of ellipticity or elliptical polarization parameters, and they are discussed in greater detail hereafter. However, it is possible, in accordance with this invention, to provide a suitably programmed calculating means which, by processing the quantities .phi. and r referred to above and the signals from which those quantities are derived, will give results which are indicative of the tilt angle, ellipticity angle and ellipticity ratio. Of course, as noted above, it is very often desirable to test the geophysical anomaly at more than one frequency; and this is accomplished without physically moving or disturbing in any way the orientation of the receiver coils, assuming that the transmitter coil also remains stationary for operation at another frequency. For these purposes, as well as for purposes of calibration and exact determination of ellipse parameters, the transmitter and receiver are each equipped with identical stable oscillators and frequency dividers. Thus, by merely switching a frequency divider setting, the frequency of the transmitted primary EM field can be altered; and, of course, a similar alteration is made to the frequency divider in the receiver so as to maintain accuracy and meaningfulness of the signal analysis.